Methods and apparatus for allocating the measured noise and resistance of a thin-film resistor between the resistor proper and the contact pads therefor

ABSTRACT

Three similar thin-film devices are fabricated in proximate relation on an insulating substrate. Each resistor has conductive contact pads fabricated thereon. The three devices may have different electrical resistances, but congruent areas underlying the contact pads are maintained for all three devices. Noise measurements and computer calculations on all three devices permit allocation of these parameters between the thin-film proper and the interfaces between the thin films and the conductive contact pads.

United States Patent Bus [151 3,692,987 [4 1 Sept. 19,1972

METHODS AND APPARATUS FOR ALLOCATING THE MEASURED NOISE AND RESISTANCEOF A THIN-FILM RESISTOR BETWEEN THE RESISTOR PROPER AND THE CONTACT PADSTHEREFOR Inventor: Laurence William Bos, West Windsor Township, MercerCounty, NJ.

Assignee: Western Electric Company, Incorporated, New York, NY.

Filed: July 6, 1970 Appl. No.: 52,568

US. Cl ..235/15l.31, 29/593, 324/57 N, 324/73 AT, 338/320 Int. Cl...G06f 15/20, G0lr 27/00 Field of Search..235/151.3, 151.31; 324/73 AT,324/57 N; 328/62, 63, 64; 338/308, 320; 29/593 References Cited UNITEDSTATES PATENTS Biard et al. ..324/73 X CONTROL CIRCUIT 3,219,927 Topp etal. ..324/73 3,458,807 7/1969 Smith ..324/73 2,911,146 ll/l959 Lanneauet al. ..235/180 2,924,384 2/1960 Porter ..235/180 3,496,461 2/1970Sessler et al. ..324/62 OTHER PUBLICATIONS Hewlett-Packard Journal Aug.1969, Vol. 20, No. 12: See page 7- 11 and 2- 6. Automated testing.

Primary Examiner-Felix D. Gruber Attorney-W. M. Kain, R. P. Miller andR. C. Winter [57] ABSTRACT Three similar thin-film devices arefabricated in proximate relation on an insulating substrate. Eachresistor has conductive contact pads fabricated thereon. The

three devices may have different electrical resistances, but congruentareas underlying the contact pads are maintained for all three devices.Noise measurements and computer calculations on all three devices permitallocation of these parameters between the thin-film proper and theinterfaces between the thin films and the conductive contact pads.

9 Claims, 4 Drawing Figures DIGITAL RESISTANCE MEASURING CIRCUIT NOISEGENERAL PURPOSE DIGITAL COMPUTER ANALOG-TO- DIGITAL CONVERTER EASURINGCIRCUIT ilh METHODS AND APPARATUS FOR ALLOCATING THE MEASURED NOISE ANDRESISTANCE OF A THIN-FILM RESISTOR BETWEEN THE RESISTOR PROPER AND THECONTACT PADS THEREFOR BACKGROUND OF THE INVENTION Broadly speaking, thisinvention relates to the measurement of noise and resistance inthin-film devices, and the like. More specifically, in a preferredembodiment, this invention relates to methods and apparatus forallocating the measured noise and resistance of a thin-film devicebetween the thin-film proper and the interface between the film andconductive contact pads which are overlaid on the film for the purposeof connecting the thin-film device to external circuitry.

With the current trend towards the miniaturization of electronicequipment, the use of thin-film devices, such as thin-film resistors andthin-film capacitors, is becoming increasingly widespread. Thin-filmresistors, for example, typically comprise a thin layer of a conductivefilm, such as tantalum nitride, which has been sputtered onto aninsulating substrate, such as ceramic. To interconnect such a devicewith other components, conductive, e.g., gold, contact pads areoverlaid, for example, by evaporation, onto the conductive film atappropriate locations thereon.

Thin-film devices, like other electrical components, inherently generateelectrical noise when a current passes therethrough. While the mechanismunderlying this phenomenon is understood for conventional components, itis not fully understood for thin-film devices. Part of the noise whichis observed in such thin-film devices is believed to originatein thethin film itself. The remainder is believed to originate in theinterface between the thin film and the conductivepads overlaid thereonto interconnect the device to the outside world. More specifically, thislatter noise is further believed to originate in oxide layer which formsbetween the tantalum nitride film and the gold contact pads. Thephysical roughness of the interface and chemiosorbed or physiosorbedimpurities sandwiched between the oxide layer and the contact padmaterial are also believed to contribute to this noise.

Ordinarily, there is no requirement to pin-point the exact locationwhere the observed noise in a thin-film device is originating. If theoverall noise figure of the device is less than some predeterminedmaximum permissible noise level, the device may be safely used for itsintended purpose. On occasion, however, it may be important to determineexactly where the noise is coming from. For example, in a manufacturingprocess, if a statistically significant number of devices consistentlyexceed the maximum permissible noise figure, it is important to knownwhether it is the tantalum nitride sputtering process used for thedeposition of terminals on the resistors, or the gold evaporationprocess that is defective, in order that appropriate corrective measuresmay be taken in regard to these processes.

Unfortunately, prior to this invention, it was not possible to make thisdetermination accurately. Thus, typically, when unacceptable deviceswere observed, it was heretofore necessary to carefully check both thesputtering and the evaporation processes for abnormal operatingconditions.

The above problem has now been solved by the instant invention which inone embodiment comprises a method of determining the noise generatedacross both the thin-film portion and the thin film-contact padinterfaces of a thin-film device. In this fashion it is possible todetermine which ofa plurality of processing steps utilized in themanufacture of the device is responsible for excessive noise observedtherein. The method comprises the steps of: fabricating at least threethin-film resistors in proximate relation on an insulating substrate;depositing a pair of conductive contact pads on each of said threeresistors, the area of the interface between said contact pads and saidthin-film resistors being the same for all three of said resistors; and,under the control of a digital computer, measuring the resistance ofsaid devices, measuring the noise voltage developed across each of saidresistors upon the application of a test potential thereto; and solvingin the computer three simultaneous equations with three unknownquantities, utilizing the results of said resistancemeasuring step andsaid noise-measuring step as a factor in said simultaneous equations,whereby the noise parameters of said thin-film portion and saidthin-film contact pad interfaces are obtained.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a typical thin-filmdevice, specifically a sputtered, tantalum-nitride, thin-film resistor;

FIG. 2 is a cross section taken about line 22 of the thin-film resistorshown in FIG. 1;

FIG. 3 is a plan view of a configuration of three such thin-film deviceswhich may be used, in accordance with this invention, to obtain thenoise and resistance parameters of the thin-film proper and the thinfilmcontact pad interface of the device shown in FIG. 1; and

FIG. 4 is a schematic representation of an apparatus which may be used,in conjunction with the thin-film configuration shown in FIG. 3, toobtain the abovementioned noise and resistance parameters.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a typicalthin-film device, specifically a thin-film resistor. As shown, thin-filmresistor 10 comprises a thin, conductive film 11, such as tantalumnitride (Ta N which has been deposited by sputtering, or otherwise, in aparticular pattern upon an insulating substrate 12, such as glass oralumina. A pair of conductive contact pads 13-13 are laid down over theextreme ends of the thin-film pattern to interconnect the resistor toexternal circuitry. Typically, pads 13-13 comprise a thin layer ofmetallic gold which has been deposited onto film 11 by an evaporationprocess.

FIG. 2 shows the layer of oxide 14 and/or other impurities inevitablypresent between the upper surface of thin film 11 and the lower surfaceof each of the contact pads 13. In FIG. 2, the dimensions of oxide layer14 are exaggerated for the purpose of clarity. Although not shown inFIG. 2, there may, in addition, be chemiosorbed or physiosorbedimpurities present between the oxide layer 14 and the lower surface ofcontact pads 13. For example, where pads 13 comprise nickel (Ni) whichhas been electrolessly deposited on a thin film of Ta N, theseimpurities might comprise palladium (Pd) atoms deposited on the oxidelayer.

Any current which passes through resistor must, of necessity, passthrough oxide layers 14 and the other impurities in addition to passingthrough the thin-film proper. Thus, the overall resistance and noisefigure of resistor 10 will have contributions from both thin film 11 andthe interfaces between pads 13 and thin film 11. Direct access to thepad-film interface is, of course, not possible, and only the overallnoise and the overall resistance of the device may be measured. Inaccordance with this invention, however, these overall measurements areused to obtain the noise and resistance contributed by the thin-filmproper and by the thin filmpad interfaces.

As is well known, the resistance of a thin-film resistor is directlypropertional to the resistivity, p, and inversely proportional to thethickness, d. It is, therefore, convenient to define a quantity R, whichis equal to p/d. The quantity, R,, is called the sheet resistance andmay be thought of as a material property since the film is essentiallytwo dimensional. A thin-film resistor consisting of a simple rectangleof length l (in the direction of the current) and width w has aresistance of (N (2) The ratio l/w is sometimes called the number ofsquares in the resistor, since it is equal to the number of squares ofside w that can be superimposed on the resistor without overlapping. Theterm squares is a pure number, having no dimensions. The sheetresistance, R,, has the units of ohms, but it is convenient to refer toit in ohms per square since the sheet resistance produces the resistanceof the resistor when multiplied v by the number of squares. The conceptcan then be broadened to include any arbitrarily shaped resistor bycalling the quantity Rd/p the effective number of squares.

FIG. 3 illustrates a special configuration of three thin-film resistorswhich may be used, in accordance with this invention, to obtain thenoise and resistance contribution of both the thin-film proper and thethin film-contact pad interface. As shown, thin-film resistor 20, 30,and 40 are deposited by sputtering, or otherwise, on insulatingsubstrate 50. Resistor 20, for example, comprises a pattern of thin-filmmaterial 21 and a pair of conductive contact pads 22-22 located ateither end of film pattern 21. Similarly, resistors and comprisepatterns of thin film 31 and 41, respectively, having conductive contactpads 32-32 and 42-42, overlaid thereon respectively.

Resistors 20, 30, and 40 are advantageously sputtered onto substrate atthe same time and in the same general area of the substrate. This willensure that the three resistors exhibit essentially identical electricaland physical properties. It will be appreciated, however, that resistors20, 30, and 40 could be formed upon a portion of some larger thin-filmcircuit or, indeed, could actually comprise components of an operatingthin-film circuit. For that matter, resistors 20, 30, and 40 need not beformed on the same substrate at the same time, but could comprise threeseparate and distinct resistors. In this event, however, care would benecessary to ensure that all three resistors exhibited essentiallysimilar electrical and mechanical properties. Since this need not bedone when all three resistors are simultaneously fabricated on the samesubstrate, this latter is a preferred embodiment. Typical resistancevalues for the resistors 20, 30, and 40 are I00 ohms, 500 ohms, and5,000 ohms, respectively.

In the configuration shown in FIG. 3, resistors 20, 30, and 40 eachpossess a different number of squares" and hence the thin-film portionsthereof, 21, 31, and 41 have different electrical resistances, R,, R andR respectively In the preferred embodiment, the path width of theresistors is assumed equal. Because the physical and electricalcharacteristics of the resistors are similar, the assumption will bemade that the specific resistivity, p, in ohm-cm of thin films 21, 31,and 41, is the same, thus p, p, p;,. Additionally, it will be assumedthat the noise generated in the thinfilm portions 21, 31, and 41 ofresistors 20, 30, and 40, respectively, can be expressed in terms of thequantity Y in p.v/volt where Y is the noise figure for any one of theresistors, but which in what follows will be assumed to be the resistorR,.

To determine the noise figures for other resistors having the same pathwidth w but a different number of squares, assume that a test potential(13,) is applied across a resistor R which has 1 square. The length andwidth of the resistive element are w, the width of the resistors in thepreferred embodiment. The resulting r.m.s. noise power is assumed to beE,,*, and the noise figure in uv/volt is given by n t) (a) where E, istaken to mean the ensemble average of the square of the noise voltage.It is next necessary to see how the noise figures of the resistorsproper are related to each other. First consider R, with n, squares. Ifad.c. potential of n,E, is applied, the current through R, and the dc.potential across each 1 square segment will be the same as in the testcase given above. Since noise voltages are uncorrelated, the noisepowers add, and the total mean squared noise voltage across R, is givenby, in (microvolts) -Ti -viii Similarly, the noise figure for R, with11, squares is WT= T E= Y: E Y

and that of R with n; squares is It will be noted that even though thesizes of the thin films 21, 31, and 41, are different, the areas andshapes of the contact pad-thin film interfaces 22, 32, and 42 areidentical for all three resistors. In other words, the area of the film21 underlying each contact pad 22 must equal the area of thin film 31underlying each contact pad 32, and so on, and, in addition, thecorresponding dimensions of the contact pads must be identical.

This latter requirement is necessary to ensure that the interfaceresistances r in ohms and the noise generating abilities of theinterfaces X in uv/volt are the same for all three resistors. Thecurrent density across the interface is greatest along the edge nearestthe resistor film. It can be shown that if this edge is doubled inlength, keeping the other dimension the same, the noise figure of theinterface will be reduced by a factor of lt will be appreciated thatmany other combinations of resistor and contact pad geometry exist whichcould be analyzed in a manner similar to that given above.

Assume now that a potential source of E volts is connected between thecontact pads of resistor 20, causing a current l to flow through thethin-film proper and the two associated thin film-contact pad interfacesof resistor 20.

Now, E=TR (8) where R R, 2r 9 and where r is the resistance of eachinterface (10) Thus, E=I(R,+2r) (It) or l=E/(R,+2r) (l2) Thus thevoltage drop across each thin film-contact pad interface, Vi is given byVi Ir Similarly, the voltage drop across the other interfaces, Vi and Viwill be given by Further, the voltage drop across the thin film 21 inresistor 20, V will be given by TE (R1+2+) (A) 2 and Vi /Y (23) 'rl',(RH-2r) m and a Vi X 25) 1'12' ('R'm'r) W 2 Analogously, the r.m.s.noise e generated in the thin films 21,31,and 41 willbe given by Now thetotal r.m.s. noise N for a plurality of series resistors, eachcontributing a noise component, 2, is given y l\l""- e, +e +e ...e,, 33Accordingly, the total noise of the resistor 20, including the thin-filmproper 21 and the two, thin film-contact pad interfaces is given by N =e+e +e 34) =2e +e (35) Similarly, for resistors 30 and 40 N =2e l-e (36)and N3 2C i 1\ (37) Substituting equations 22 and 28 in equation 35 (N/E )(R,+2r) =2X r WR, (39) Similarly,

(Ni/E (R -l-2r) =2X r WR,R 40

(Ni/E (R +2r) =2X r l R R 4|) Now, E is known, and N N and N, can bemeasured with a noise-measuring test set. Further R R and R can bemeasured with an ohmmeter connected between the contact pads. Such aresistance measurement will include the resistance of the two interfaces(2r) but the magnitude of the interface resistance is so small withrespect to R1, R2, or R3, that, for practical purposes, it can beignored during direct measurement of R1, R2, or R3. Thus, r may besolved for in terms of N and R etc. by eliminating X and Y. EliminatingX from equations 40 and 41 yields:

Eliminating Y from equations 42 and 43 yields:

By collecting and rearranging terms, equation 44 can be written as:

This equation can be written more simply by defining This equation canbe solved by standard algebraic techniques to give 70. Output circuitsof the digital computer 70 are connected to a control circuit 71 whichcontrols the switching matrix 61.

The switching matrix 61 may comprise any number of well-known switchingdevices, but advantageously is a group of six relays; such as relay 72,having a coil 73 which operates a single, normally open contact 74, withthe contacts, such as contact 74, connected in a 2 X 3 matrix. The relaycontacts 74 are preferably gold or silver plated to provide a low-noiseand low-resistance path when a contact is closed, to thereby minimizeadditional resistance and noise during the performance of the resistanceand noise measurements on the resistors 20, 30, and 40. The controlcircuit 71 advantageously comprises a bistable circuit and a driveramplifier for each relay, so that each relay can be operated andreleased by pulses from the digital computer 70. Such a control circuitis well known in the art.

The resistance measuring circuit 62, the noise measuring circuit 65, theanalog-to-digital converter 66, and the digital computer 70 can becommercially available units. For example, the digital resistancemeasuring circuit 62 can be a Fluke Model 8300A Digital Voltmeter withOhmmeter Accessory, the noise measuring circuit 63 can be a Quan-TechModel 315 Resistor-Noise Test Set, the analog-to-digital converter canbe a Digital Equipment Corporation Model ADO8- A, and the digitalcomputer can be a Digital Equipment Corporation PDP8I. Theinterconnection details of apparatus,,such as is shown in FIG. 4, arediscussed in the Digital Equipment Corporation Small Computer Handbook1970 Edition, beginning on page 229. The Small it can be shown (forexample, by letting Y 0) that, in Computer Handbook also describes theoperation of general, the coefficients of r and r in equation have thesame sign while the constant term has a sign opposite to that of thefirst two coefficients. Thus, the plus sign is required in front of theradical in equation to arrive at a physically meaningful value for r.

Now that r has been determined, only two dependent variables remain, andany two of the equations 39, 40, and 41 can be used to solve for X and YAccordingly, 39 and 40 for R and R will be used. The result is FIG. 4depicts one illustrative apparatus which may be used, according to thisinvention, to automatically deter-mine the noise and resistancecontribution of the thin film and thin film-contact pad interfaces ofresistors 20, 30, and 40.

One contact of each of the resistors 20, 30, and 40 is connected toground. The other contact of each of the resistors 20, 30, and 40 isconnected through a switching matrix 61 to either a digital resistancemeasuring circuit 62, or by a lead 63 to a source of test potential 64and a noise measuring circuit 65. The analog voltage output of the noisemeasuring circuit 65 is connected to an analog-to-digital converter 66.The digital outputs from the digital resistance measuring circuit 62,and from the analog-to-digital converter 66, are connected to inputcircuits of a digital computer the PDP81 computer, beginning on page 1.

If a Quan-Tech Resistor-Noise Test Set is used for the noise measuringcircuit 65, the separate potential source 64 is not necessary, as thistest set includes a potential source equivalent to the potential source64. The separate potential source 64 is shown in FIG. 4, however, thefacilitate explanation.

in operation, digital computer controls the switching matrix 61 toconnect resistors 20, 30, and 40, in turn, to the digital resistancemeasuring circuit 62, so that the resistance of resistors 20, 30, and 40can be measured. After a resistor has been measured, the resultingdigital resistance value is tranferred into the digital computer 70, andis stored in the computer memory for 'use in subsequent calculations.The digital computer 70 then controls the switching matrix 61 to connectresistors 20, 30, and 40, in turn, to the potential source 64 and thenoise measuring circuit 65 so that the noise voltage generated by eachresistor can be measured. The voltage, E, supplied by the potentialsource 64, causes a current to flow through the resistor being tested.The noise voltage thereupon generated by the resistor is measured by thenoise measuring circuit 65, which transmits an analog voltage signal,representative of the magnitude of the measured noise voltage, to theanalog-to-digital converter 66. The analog-todigital converter 66converts the analog voltage signal representative of the measured noiseinto digital form suitable for transfer into the digital computer 70.After a noise measurement has been made, the resulting digital noisevalue is stored in the computer memory for use in subsequentcalculations.

In order to clarify the actions of the digital computer 70 in performingthe methods of the invention with the apparatus shown in FIG. 4, asequence of computer program steps will be described. The steps setforth a series of operations of the digital computer which can beperformed to determine the desired noise factors X and Y for a set ofthree resistors on a substrate under test. The steps can easily beencoded into computer instructions for the PDP8I computer, or anysimilar computer, by one skilled in the programming art.

The computer program steps are as follows:

Step 1: Control matrix switch 61 to connect resistor 20 to digitalresistance measuring circuit 62; input and store digital resistancemeasurement, R for resistor 20.

Step 2: Control matrix switch 61 to connect resistor 30 to digitalresistance measuring circuit 62; input and store digital resistancemeasurement, R for resistor 30.

Step 3: Control matrix switch 61 to connect resistor 40 to digitalresistance measuring circuit 62; input and store digital resistancemeasurement, R for resistor 40.

Step 4: Control matrix switch 61 to connect resistor 20 to potentialsource 64 and noise measuring circuit 65; input and store digital noisemeasurement N for resistor 20.

' Step 5: Control matrix switch 61 to connect resistor 30 to potentialsource 63 and noise measuring circuit 65; input and store digital noisemeasurement N for resistor 30.

Step 6: Control matrix switch 61 to connect resistor 40 to potentialsource 64 and noise measuring circuit 65; input and store digital noisemeasurement N for resistor 40.

Step 7: Calculate the value of r by evaluating the expression:

scope of the invention to films which have been deposited onto asubstrate by cathodic sputtering or by evaporation, which, to some inthe art, are called thin films regardless of their actual physicaldimensions. Thus, the invention has application to the so-called thickfilms," i.e., films that have been deposited mechanically, or otherwise,onto a substrate, e.g., ceramic, and then fired into the substrate by aheat process, or the like. Such thick films" may in some instances bethinner than the so-called thin films. In any event, the term thin film,as used in the specification and claims, is intended to cover all suchfilms regardless of relative thickness and/or the manner of depositionand generally means a metal or metallic film which is at least smallerin thickness than the underlying substrate which supports it.

Further, the term deposit, as used in the specification and claims, withreference to the creation of the interface between the thin-film properand the conductive contact pads, is not used in a limiting sense butencompasses sputtering, evaporation, silk-screening, etc.

One skilled in the art can make various changes and modifications to themethods and apparatus disclosed herein without departing from the spiritand scope of this invention.

What is claimed is: 1. A method of determining electrical noise figuresfor the thin-film portions and the thin film-contact pad interfaces ofthin-film devices fabricated on an insulating substrate, comprising thesteps of:

fabricating three thin-film resistors, having different resistances, inproximate relation on the substrate;

depositing a pair of conductive contact pads on each of said resistors,the areas of the interfaces between said contactpads and said resistorsbeing equal for all of said resistors;

by automatic measuring means, determining the resistance between thecontact pads of each of said resistors, and storing the values of theresistances; by automatic measuring means, determining the where l 1 23) 2 N22(R3 R1) s s 1' 2) Step 8: Calculate the value of X by evaluatingthe expression:

The value of E has previously been stored in the computer memory.

Step 9: Calculate the value of Y by evaluating the expression:

- Step 10: Output of the values of r, X and Y.

The values of r, X and Y calculated by the digital computer 70 can betransmitted to any suitable output device (not shown), such as a printeror a visual disla p li should be appreciated that the term thin film,"as used in the specification and claims, is not used in any limitingsense. I specifically do not intend to limit the noise voltage generatedby each of said resistors during the application of a test potentialbetween the contact pads thereof, and storing the values of the noisevoltages; and

by machine means, solving the equations:

Y is the noise figure for the thin-film portions of the resistors.

2. A method of operating a digital computer to determine electricalnoise figures for the thin-film portions and thin film-contact padinterfaces of thin-film devices fabricated on a substrate, wherein threethin-film resistors, having different resistance values, are fabricatedon said substrate, and a pair of conductive contact pads is deposited oneach of said resistors, with the areas of the interfaces between saidcontact pads and said resistors being equal for all of said resistors,which comprises:

under the control of said computer, initiating a measurement of theresistance between the contact pads of each of said resistors, andstoring the resulting resistance values, R R and R in the computer;

under the control of said computer, initiating a measurement of thenoise voltages generated by each of said resistors during theapplication of a test potential E between the contact pads thereof, andstoring the values of the noise voltages N N and N corresponding to R Rand R respectively; and

solving, in said computer, the equations:

for unknowns r, X, and Y, the value ofE being previously stored in thecomputer, and where:

r is the resistance of each of the contact pad-thin film interfaces;

X is the noise figure for the contact pad-thin film interfaces; and

Y is the noise figure for the thin-film portions of the resistors.

3. Apparatus for allocating the electrical noise generated by thin-filmdevices, fabricated on a substrate, between the thin-film portion andthe thin filmcontact pad interfaces, comprising:

three thin-film resistors fabricated on said substrate,

having different resistances R R and R and having a pair of conductivecontact pads fabricated thereon, the area of thinfilm materialunderlying each of the contact pads being the 4. A process for using adigital computer to determine the electrical noise generated across boththe thin-film portions and the thin film-contact pad interfaces ofthin-film devices fabricated on a substrate, comprising the steps of:

generating first digital signals representative of the resistances ofthree thin-film resistors fabricated on said substrate, each of saidresistors possessing a different resistance, and having two conductivecontact pads affixed thereto, the areas of the thinfilm materialunderlying each of said contact pads being the same for each of saidresistors;

generating second digital signals representative of the noise voltagesgenerated across each of said resistors during the application of a testpotential thereacross; and

by machine means, operating on said first and second digital signals,and a digital signal representative of said test potential, to derivedigital signals representative of the noise contributions of thethinfilm portions and the thin film-contact pad interfaces of saiddevices.

5. A method of using a digital computer to allocate the electrical noisegenerated by thin-film devices fabricated on a substrate between thethin-film portions and the interfaces between the thin film andconductive contact pads overlaid on the film, comprising the steps of:

fabricating three electrically and physically similar thin-filmresistors on adjacent regions on said substrate, each of said resistorshaving two conductive contact pads overlaid thereon, the total thin-filmarea contacted by each of the pads being substantially equal for eachone of said at least three resistors;

under the control of said computer, measuring the electrical resistancesR,, R and R of each of said resistors, and storing the values of saidresistances in the computer;

under the control of said computer, measuring the electrical noisevoltages N N and N generated by each of said resistors during theapplication thereacross of a test potential E, and storing the values ofsaid noise voltages in the computer;

in said computer, calculating the resistance r of the contact pad-thinfilm interfaces according to the formula:

same for each of said resistors;

a digital computer;

means, under the control of said digital computer, for measuring theresistance of said resistors and for transmitting said resistance valuesto the computer for storage therein;

a source of test potential;

means, under the control of said digital computer, for measuring thenoise voltages generated by each of said resistors during theapplication of said test potential, and for transmitting the values ofsaid noise voltages to the computer for storage therein; said computerperforming calculations using said stored resistance values and noisevalues to determine the desired noise allocation.

where and;

in said computer, calculating thenoise X allocable to the contactpad-thin film interfaces on said devices according to the formula:

where the value of E has previously been stored in said computer.

6. A method of determining the noise generated across both the thin-filmportion and the thin film-contact pad interfaces of a thin-film device,to aid in determining which of the plurality of processing stepsutilized in the manufacture of the device is responsible for excessivenoise observed therein, comprising the steps of:

fabricating three thin-film resistors in proximate relation on aninsulating substrate utilizing the same processes which were used tomanufacture the device observed to have excessive noise;

depositing a pair of conductive contact pads on each of said threeresistors, the areas of the interfaces between said contact pads andsaid thin-film portions being the same for all of said devices;

measuring, under the control of a digital computer, the electricalresistances of each of said resistors and storing the resultingresistance values R R and R in the digital computer;

measuring, under the control of the digital computer,

the noise voltages generated by each of said resistors during theapplication of a test potential E across the contact pads thereof, andstoring the values of the noise voltages corresponding to R R and R, asN N and N respectively;

and

solving, in said computer, the equations:

for unknowns r, X, and Y, the value of E being previously stored in thecomputer,

where:

r is the resistance of each of the contact pad-thin film interfaces ofthe resistors;

X is the noise figure for the contact pad-thin film interfaces of theresistors;

Y is the noise figure for the thin-film portions of the resistors,whereby comparisons with desired values of X and Y can be made todetermine whether the excessive noise is being caused by the thin-filmfabrication steps or by the contact pad fabrication steps.

7. A method of operating a digital computer to determine the noisegenerated across both the thin-film portion and the thin film-contactpad interfaces of a thinfilm device, to determine which of a pluralityof fabricating steps utilized in the manufacture of the device isresponsible for excessive noise observed therein, using a thin-filmcircuit comprising three thinfilm resistors fabricated in proximaterelation on an insulating substrate by the same processes which wereused to manufacture the device observed to have excessive noise, each ofsaid resistors having a pair of conductive contact pads depositedthereon, the area of the interface between the contact pads and saidthin film being the same for all three of said resistors, comprising thesteps of:

under the control of said computer, measuring the electrical resistanceof each of said resistors and storing the resulting resistance values, RR and R in the digital computer; under the control of said computer,measuring the noise voltages generated by each of said resistors duringthe application of a test potential E across the contact pads thereof,and storing the values of the noise voltages corresponding to R R and Ras N N and N respectively; and solving, in said computer, the equations:

for unknowns r, X, and Y, the value of E being previously stored in thecomputer,

where:

r is the resistance of each of the contact pad-thin film interfaces ofthe resistors;

X is the noise figure for the contact pad-thin film interfaces of theresistors;

Y is the noise figure for the thin-film portions of the resistors,whereby comparisons with desired values of X and Y can be made todetermine whether the excessive noise is being caused by the thin-filmfabrication steps or the contact pad fabrication steps.

8. A process for determining, by the use of a digital computer toanalyze the noise generated across both the thin-film portion and thethin film-contact pad interfaces of a thin-film device, which of aplurality of processing steps utilized in the manufacture of thethinfilm device is primarily responsible for an unacceptably high noiselevel in the device, comprising the steps of:

generating, under the control of said digital computer, first digitalsignals representative of the resistances of three thin-film resistorsfabricated on said substrate, each of said resistors possessing adifferent resistance, and having two conductive contact pads affixedthereto, the areas of the thinfilm material underlying each of saidcontact pads being the same for each of said resistors;

generating, under the control of said digital computer, second digitalsignals representative of the noise voltages generated across each ofsaid resistors during the application of a test potential thereacross;and

in said digital computer, operating on said first and second digitalsignals, and a digital signal representative of said test potential, toderive digital signals representative of the noise contributions of thethin-film portions and the thin film-contact pad interfaces of saiddevices, whereby comparisons with digital signals representing optimumnoise contributions can be made to determine whether the excessive noiseis being caused by the thin-film processing steps or the contact padprocessing steps.

9. A method for determining the noise contributions in a thin-filmresistor, and its interfaces with external circuitry, comprising thesteps of:

fabricating three thin-film resistors having different resistances inproximate relation on an insulating substrate;

attaching conductive means to said resistors which provide identicalinterface areas between said conductive means and each of saidresistors;

under the control of machine means, measuring the resistance of each ofsaid resistors and storing in said machine means the resistance values RR and R under the control of machine means, measuring the noise voltagesgenerated by each of said resistors during the application of a testpotential, E, thereacross, and storing in said machine means the valuesof the noise voltages corresponding to R,, R and R; as N N and Nrespectively; and by machine means, solving, the equations:

UNITED STATES PATENT OFFICE CERTIFEQATE 0F at: EUH

patent 3, 92,9 7 Dated September 19, 1972 Inventor) Laurence William BosIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

[- In the specification, Column 1, line 39, in oxide" should I read--in. the oxidelines 56-58, "sputtering process used for the depositionof terminals on the resistors, or the gold evaporation process that isdefective, should read --sputtering process, used for the fabrication ofthe resistors, or the gold evaporation process, used for the depositionof terminals on the resistors that is defective,-- I Column 5, equation8, "E TR" should read --E -IR Column 6, equation 32 RE I RE R 3 1 (Y)should read 3 1/ (Y) v R +2r 1T I 3 3 Column 7, equation #2 Y R E 2 2 21 R +2r N 2 R +2r N 1" should read E 'Z'R' 2 2 3 3 Column 7, equation LB"Y R E 1 2 2 2 2 R +2 N R +2 H i -R 1 r) 1 3 should read 2 2 l I 2 2 i 22 Y R E I 4 ll z i' Tj 1 (R +2r) N3 1 UNITED STATES PATENT OFFICE,"'CE'RT-IFKATE 0F CORRECTION lpat'emNo 3, 9 ,9 7 Dated September 19,1972 lnvemods) Laurence William Bos It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

ziolumn 7, equation 44, 1

Oolumi'iv 7, last line of equation L5- 2 2 +3 N R R 3 (R R O should readN (R3-R R N r 1/2( [Rd R B should read I ow 3 1 1 2 2 3 3 I v LfSSG-PT-(6- .9)

UNITED STATES PATENT oFmcE CERTIFICATE OF CORRECTION Paten -N I DatedSeptember 19, 1972 Inv Laurence William Bos It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

zolumn 73 line 51, insert equation number (5l)--. Column 8,

line 1+7 "the" should read --to--. Column 9, line 33, "63" should read--6 -L-- line #2 1/2 1 R R R should readl 2 1 Roz R R q (M 2 3 1 1 22 33 Column 9, line 62, "Step 10; Output of the" should read --St .ep 10:Output the-- Column 12, claim 5, line 52 H l R X R should read In theclaimsj Column 13, claim 6, line 37 2 2 2 r "2E r X E iR Y M 1 lrR N N Rshould read 2 2 2 2 2 2 2 2 2 2 2 --2E r X E R Y mi 1' LI'R N N1 R1Column 1 elaim 7, line 17 2E r X E 3 Y 4N1 1r LrR N N R1 1 should read i-2 E r X E R 2Y2 LN r ArR N N 2R 2 Signed and sealed this 10th day ofApril 1973.

(SEA-L) I Attest:

EDWARD I LFLETCHER,JR.

7 ROBERT GOTTS A-tt-esting Officer CHALK Commissioner of Patents

1. A method of determining electrical noise figures for the thin-filmportions and the thin film-contact pad interfaces of thin-film devicesfabricated on an insulating substrate, comprising the steps of:fabricating three thin-film resistors, having different resistances, inproximate relation on the substrate; depositing a pair of conductivecontact pads on each of said resistors, the areas of the interfacesbetween said contact pads and said resistors being equal for all of saidresistors; by automatic measuring means, determining the resistancebetween the contact pads of each of said resistors, and storing thevalues of the resistances; by automatic measuring means, determining thenoise voltage generated by each of said resistors during the applicationof a test potential between the contact pads thereof, and storing thevalues of the noise voltages; and by machine means, solving theequations: 2E2r2X2 + E2R12Y2 4N12r2 - 4rR1N12 N12R12 2E2r2X2 +E2R1R2Y2 - 4N22r2 - 4rR2N22 N22R22 2E2r2X2 + E2R1R3Y2 - 4N32r2 - 4rR3N32N32R32 for unknowns r, X, and Y, where: R1, R2, and R3 are theresistance values measured in said resistance determining step; N1, N2,and N3 are the noIse voltage values measured in said noise determiningstep; E is the magnitude of the test potential which has been previouslystored in the computer; r is the resistance of each of the thinfilm-contact pad interfaces; X is the noise figure for the thinfilm-contact pad interfaces; and Y is the noise figure for the thin-filmportions of the resistors.
 2. A method of operating a digital computerto determine electrical noise figures for the thin-film portions andthin film-contact pad interfaces of thin-film devices fabricated on asubstrate, wherein three thin-film resistors, having differentresistance values, are fabricated on said substrate, and a pair ofconductive contact pads is deposited on each of said resistors, with theareas of the interfaces between said contact pads and said resistorsbeing equal for all of said resistors, which comprises: under thecontrol of said computer, initiating a measurement of the resistancebetween the contact pads of each of said resistors, and storing theresulting resistance values, R1, R2, and R3, in the computer; under thecontrol of said computer, initiating a measurement of the noise voltagesgenerated by each of said resistors during the application of a testpotential E between the contact pads thereof, and storing the values ofthe noise voltages N1, N2, and N3, corresponding to R1, R2, and R3,respectively; and solving, in said computer, the equations: 2E2r2X2 +E2R12Y2 -4N12r2 - 4rR1N12 N12R12 2E2r2X2 + E2R1R2Y2 - 4N22r2 -4rR2N22N22R22 2E2r2X2 + E2R1R3Y2 - 4N32r2 - 4rR3N32 N32R32 for unknowns r, X,and Y, the value of E being previously stored in the computer, andwhere: r is the resistance of each of the contact pad-thin filminterfaces; X is the noise figure for the contact pad-thin filminterfaces; and Y is the noise figure for the thin-film portions of theresistors.
 3. Apparatus for allocating the electrical noise generated bythin-film devices, fabricated on a substrate, between the thin-filmportion and the thin film-contact pad interfaces, comprising: threethin-film resistors fabricated on said substrate, having differentresistances R1, R2, and R3, and having a pair of conductive contact padsfabricated thereon, the area of thin-film material underlying each ofthe contact pads being the same for each of said resistors; a digitalcomputer; means, under the control of said digital computer, formeasuring the resistance of said resistors and for transmitting saidresistance values to the computer for storage therein; a source of testpotential; means, under the control of said digital computer, formeasuring the noise voltages generated by each of said resistors duringthe application of said test potential, and for transmitting the valuesof said noise voltages to the computer for storage therein; saidcomputer performing calculations using said stored resistance values andnoise values to determine the desired noise allocation.
 4. A process forusing a digital computer to determine the electrical noise generatedacross both the thin-film portions and the thin film-contact padinterfaces of thin-film devices fabricated on a substrate, comprisingthe steps of: generating first digital signals representative of theresistances of three thin-film resistors fabricated on Said substrate,each of said resistors possessing a different resistance, and having twoconductive contact pads affixed thereto, the areas of the thin-filmmaterial underlying each of said contact pads being the same for each ofsaid resistors; generating second digital signals representative of thenoise voltages generated across each of said resistors during theapplication of a test potential thereacross; and by machine means,operating on said first and second digital signals, and a digital signalrepresentative of said test potential, to derive digital signalsrepresentative of the noise contributions of the thin-film portions andthe thin film-contact pad interfaces of said devices.
 5. A method ofusing a digital computer to allocate the electrical noise generated bythin-film devices fabricated on a substrate between the thin-filmportions and the interfaces between the thin film and conductive contactpads overlaid on the film, comprising the steps of: fabricating threeelectrically and physically similar thin-film resistors on adjacentregions on said substrate, each of said resistors having two conductivecontact pads overlaid thereon, the total thin-film area contacted byeach of the pads being substantially equal for each one of said at leastthree resistors; under the control of said computer, measuring theelectrical resistances R1, R2, and R3 of each of said resistors, andstoring the values of said resistances in the computer; under thecontrol of said computer, measuring the electrical noise voltages N1,N2, and N3 generated by each of said resistors during the applicationthereacross of a test potential E, and storing the values of said noisevoltages in the computer; in said computer, calculating the resistance rof the contact pad-thin film interfaces according to the formula: Alpha1 N12(R2 - R3) Alpha 2 N22(R3 - R1) Alpha 3 N32(R1 - R2) in saidcomputer, calculating the noise Y allocable to the thin-film portions ofsaid devices according to the formula: and; in said computer,calculating the noise X allocable to the contact pad-thin filminterfaces on said devices according to the formula: X Square Root(1/2r2) ((N12/E2) (R1 + 2r)2 -Y2R12) where the value of E has previouslybeen stored in said computer.
 6. A method of determining the noisegenerated across both the thin-film portion and the thin film-contactpad interfaces of a thin-film device, to aid in determining which of theplurality of processing steps utilized in the manufacture of the deviceis responsible for excessive noise observed therein, comprising thesteps of: fabricating three thin-film resistors in proximate relation onan insulating substrate utilizing the same processes which were used tomanufacture the device observed to have excessive noise; depositing apair of conductive contact pads on each of said three resistors, theareas of the interfaces between said contact pads and said thin-filmportions being the same for all of said devices; measuring, under thecontrol of a digital computer, the electrical resistances of each ofsaid resistors and storing the resulting resistance values R1, R2, andR3 in the digital computer; measuring, under the control of the digitalcomputer, the noise voltages generated by each of said resistors duringthe application of a test potential E across the contact pads thereof,and storing the values of the noise voltages corresponding to R1, R2,and R3 as N1, N2, and N3, respectively; and solving, in said computer,the equAtions: 2E2r2X2 + E2i R12Y2 -4N12r2 - 4rR1N12 N12R12 2E2r2X2 +E2R1R2Y2 - 4N22r2 - 4rR2N22 N22R22 2E2r2X2 + E2R1R3Y2 - 4N32r2 - 4rR3N32N32R32 for unknowns r, X, and Y, the value of E being previously storedin the computer, where: r is the resistance of each of the contactpad-thin film interfaces of the resistors; X is the noise figure for thecontact pad-thin film interfaces of the resistors; Y is the noise figurefor the thin-film portions of the resistors, whereby comparisons withdesired values of X and Y can be made to determine whether the excessivenoise is being caused by the thin-film fabrication steps or by thecontact pad fabrication steps.
 7. A method of operating a digitalcomputer to determine the noise generated across both the thin-filmportion and the thin film-contact pad interfaces of a thin-film device,to determine which of a plurality of fabricating steps utilized in themanufacture of the device is responsible for excessive noise observedtherein, using a thin-film circuit comprising three thin-film resistorsfabricated in proximate relation on an insulating substrate by the sameprocesses which were used to manufacture the device observed to haveexcessive noise, each of said resistors having a pair of conductivecontact pads deposited thereon, the area of the interface between thecontact pads and said thin film being the same for all three of saidresistors, comprising the steps of: under the control of said computer,measuring the electrical resistance of each of said resistors andstoring the resulting resistance values, R1, R2, and R3, in the digitalcomputer; under the control of said computer, measuring the noisevoltages generated by each of said resistors during the application of atest potential E across the contact pads thereof, and storing the valuesof the noise voltages corresponding to R1, R2, and R3 as N1, N2, and N3,respectively; and solving, in said computer, the equations: 2E2r2X2 +E2R12Y2 -4N12i r2 - 4rR1N12 N12R12 2E2r2X2 + E2R1R2Y2 - 4N22r2 -4rR2N22N22R22 2E2r2X2 + E2R1R3Y2 - 4N32r2 - 4rR3N32 N32R32 for unknowns r, X,and Y, the value of E being previously stored in the computer, where: ris the resistance of each of the contact pad-thin film interfaces of theresistors; X is the noise figure for the contact pad-thin filminterfaces of the resistors; Y is the noise figure for the thin-filmportions of the resistors, whereby comparisons with desired values of Xand Y can be made to determine whether the excessive noise is beingcaused by the thin-film fabrication steps or the contact pad fabricationsteps.
 8. A process for determining, by the use of a digital computer toanalyze the noise generated across both the thin-film portion and thethin film-contact pad interfaces of a thin-film device, which of aplurality of processing steps utilized in the manufacture of thethin-film device is primarily responsible for an unacceptably high noiselevel in the device, comprising the steps of: generating, under thecontrol of said digital computer, first digital signals representativeof the resistances of three thin-film resistors fabricated on saidsubstrate, each of said resistors possessing a different resistance, andhaving two conductive contact pads affixed thereto, the areas of thethin-film material underlying each of said contact pads being the samefor each of said resistors; generating, under the control of saiddigital computer, second digital signals representative of the noisevoltages generated across each of said resistors during the applicationof a test potential thereacross; and in said digital computer, operatingon said first and second digital signals, and a digital signalrepresentative of said test potential, to derive digital signalsrepresentative of the noise contributions of the thin-film portions andthe thin film-contact pad interfaces of said devices, wherebycomparisons with digital signals representing optimum noisecontributions can be made to determine whether the excessive noise isbeing caused by the thin-film processing steps or the contact padprocessing steps.
 9. A method for determining the noise contributions ina thin-film resistor, and its interfaces with external circuitry,comprising the steps of: fabricating three thin-film resistors havingdifferent resistances in proximate relation on an insulating substrate;attaching conductive means to said resistors which provide identicalinterface areas between said conductive means and each of saidresistors; under the control of machine means, measuring the resistanceof each of said resistors and storing in said machine means theresistance values R1, R2, and R3; under the control of machine means,measuring the noise voltages generated by each of said resistors duringthe application of a test potential, E, thereacross, and storing in saidmachine means the values of the noise voltages corresponding to R1, R2,and R3 as N1, N2, and N3, respectively; and by machine means, solving,the equations: 2E2r2X2 + E2R12Y2 -4N12r2 - 4rR1N12 N12R12 2E2r2X2 +E2R1R2Y2 - 4N22r2 - 4rR2N22 N22R22 2E2r2X2 + E2R1R3Y2 - 4N32r2 - 4rR3N32N32R32 for unknowns r, X, and Y, the value of E being previously storedin said machine means, where r represents the resistance of each of thecontact pad-thin film interfaces, and X and Y represent, respectively,the noise figure of said interface areas and that of said resistorsexternal to said interface areas.